from PIL import Image
img = Image.open("galaxy-full.jpg")
pixels = img.load()
w, h = img.size
for x in range(int(w/2), w) :
    for y in range(int(h/2)):
        _, g, _ = pixels[x, y]
        pixels[x, y] = 0, g, 0
img.show()
img.save("galaxy-green.jpg", "JPEG")